On the Stability of Amino Acid Zwitterions in the Gas Phase: The Influence of Derivatization, Proton Affinity, and Alkali Ion Addition

Collision cross sections have been measured for a series of N- and C-methylated glycines cationized by alkali ions using ion mobility methods. In all cases the measured cross sections are in excellent agreement with model structures obtained from a number of different theoretical approaches. Unfortunately both charge solvation and zwitterion structures are predicted to have nearly identical cross sections. On the basis of a conformational search by molecular mechanics methods and density functional theory calculations at the B3LYP/DZVP level it is found that the lowest energy forms of alkali cationized glycine and alanine are charge solvation structures, whereas lowest energy singly and doubly N-methylated glycines are salt bridges independent of metal ion. α-Amino isobutyric acid forms a salt bridge when sodiated and a charge solvation structure when rubidiated. In the most stable charge solvation structures rubidium is bound to one or both carboxyl oxygens, while sodium is bound to both the N- and the C...

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